There is a growing interest in the application of low coherence interferometry in the general field of sensing. Low coherence interferometry methods provide absolute distance measurements and are well suited for measuring absolute or relative distances based on signal returned by rough reflecting surfaces. Spectral low coherence interferometry (LCI) methods are based on the measurement of periodicity of the channelled spectrum of the optical signal coming from a two beam interferometer. The larger the optical path difference (OPD) in the interferometer, the denser the spectral modulation of the channelled spectrum. This can be read using a spectrometer, employing a dispersing element, such as a prism or a diffraction grating, to disperse respectively diffract light on a linear photodetecting camera to transduce the channelled spectrum of the interferometer output into a temporal signal, when the interferometer is excited by a large bandwidth optical source. Alternatively, a narrow band tuneable optical source, a swept source (SS), can be employed. By tuning the optical frequency of the optical source, the channelled spectrum is scanned point by point and a temporal signal is obtained again.
Channelled spectrum methods have been used in the sensing and fibre optic sensing field. Several implementations are known, using photodetector linear arrays, such as CCD and CMOS, to interrogate the optical signal output of the sensing interferometer, which allows to scan the channelled spectrum and produce a measuring signal. Such a method and device are disclosed in “Channeled Spectrum Display using a CCD Array for Student Laboratory Demonstrations”, published by A. Gh. Podoleanu, S. Taplin, D. J. Webb and D. A. Jackson in the European J. Phys., 15, (1994), p. 266-271.
The advantage of spectral methods is that the OPD information is translated into the periodicity of peaks and troughs in the channelled spectrum and no mechanical means are needed to scan the object in depth, when performing optical coherence tomography (OCT) of tissue.
If multi-layered objects are imaged, such as tissue, each layer will imprint its own channelled spectrum periodicity, depending on its depth, with the amplitude of the spectrum modulation proportional to the square root of the reflectivity of that layer. A fast Fourier transform (FFT) of the signal delivered by a linear photodetector array, a CMOS or CCD linear camera signal, translates the periodicity of the channelled spectrum into peaks of different frequencies, with the frequency directly related to the OPD value. This measurement method is called frequency domain LCI (FD-LCI). The reflectivity profile with depth obtained by FFT is termed as an A-scan. Grouping together several A-scans, a B-scan or a cross section OCT image is obtained.
If a SS is used to scan the channelled spectrum of the interferometer, then the channelled spectrum profile is obtained directly in time, as a signal delivered by a photodetector device, method called SS-LCI. The FFT of such a signal leads to an A-scan again.
The methods above present the disadvantage that information is obtained by performing FFT. This requires a processor or a PC. Also, the standard method requires a display device, usually a monitor of a PC or a Laptop. Despite the continuous progress in computing and digital signal processing, these systems and devices raise the size and cost of FD-LCI and SS-LCI systems and of their OCT counterparts, FD-OCT and SS-OCT systems.
In measurements of distances in the field, in constructions, industry, portable systems are required. To extend spectral domain—LCI measurements to such sensing and industrial applications, low cost, small size and reduced weight systems are necessary.
In ophthalmology, measurement of eye length is performed before any cataract operation. Such measurements are performed using high cost instruments. Such instruments have a large size and are expensive. There is a need for such measurements to be more accessible to small ophthalmology practices. There is also a proven need to liaise the audio signal to the value of a quantity to be measured in complex environments where the sight is concentrated on the most complex tasks, such as surgery.
The patent application US2005/023727 A1, by Podoleanu and Rogers, used a loudspeaker to indicate the strength of the interference signal in a time domain optical coherence tomography system. The audible signal strength was an indication of signal detected and was not used in any measurement of any quantity.
Patent application US2008/0218588 A1, proposes an audio signal to transmit information about a captured image. However, this audio signal is used for transmission means only and does not allow for the direct monitoring or measurement of a system parameter.
The present invention provides methods and apparatuses which can advantageously perform measurements of lengths and optical path difference using a minimum of devices which can be conveniently assembled in a small size, low weight and low cost instrument that can be operated independent of computational power, simply by following a meter indication, a needle, a digital indication, a source of light or a source of sound.